* First try making get_recipients use MailHeader (nice and functional)
* Get it to compile by using not-so-functional style
* Add "empty-from" test, drop unnecessary check for error; continue using addrparse_header() instead of addrparse()
* Try to use functional style, unfortunately, I can't get the compiler to accept it
* Do it imperative-style: Do not overwrite To with Cc and vice versa
* Use addrparse_header() once more
* Still addrparse_header()
* Clippy
* Fix compile errors in tests
* Fix typo
* Fix tests again ;-)
* Code style
* Code style; try a HashMap<addr: String, display_name: String> as an address list but I am not convinced
* Code style; Use Vec<SingleInfo> as address list
* Clippy
* Add tests
* Add another test
* Remove stale comments
we change the name on the wire as the old Chat-Group-Image header
could not be used on random mails, it was the marker for a "Changed" message,
if we would keep this names, things will fail for exising installations
as messages are dropped and a "Group image changed" message is shown instead.
